This disclosure pertains to a process of making aqueous pigment dispersions. These dispersions may be used in the preparation of aqueous ink-jet inks, pigmented paints, and the like. More particularly, this disclosure relates to a process of making aqueous pigment dispersions by coating the pigments with coating agents using a grinding process.
Aqueous dispersions of pigments are widely used in ink-jet printing. Because a pigment is typically not soluble in an aqueous vehicle, it is often required to use a dispersing agent, such as a polymeric dispersant or a surfactant, to produce a stable dispersion of the pigment in the aqueous vehicle.
The use of dispersing agents, however, increases the viscosity of the dispersion over what it would be without the dispersing agents. Certain end use applications of the dispersion, such as ink-jet ink, may have limitations on viscosity, thus having dispersing agents in a dispersion may reduce the formulation latitude of the dispersion for final products.
The use of self-dispersing pigment has been known in the art for many years. U.S. Pat. No. 2,439,442 discloses a process in which a carbon black pigment is exothermically reacted with a water solution of sodium hypochlorite, or is subjected to electrolysis in a sodium chloride solution, or is suspended in a sodium hydroxide solution and treated with chlorine gas to alter the colloidal properties such that the carbon black will readily and spontaneously disperse in water. U.S. Pat. No. 6,852,156 discloses a process of oxidizing carbon black using ozone under a dispersive environment. U.S. Pat. No. 6,120,594 discloses the oxidation of carbon black with hydrogen peroxide.
While there are many approaches focusing on modifying the pigment surface by chemical reactions leading to hydrophilic moieties covalently attached to the pigment surface, organic color pigments such as quinacridone magenta, cyan and yellow are inherently difficult to treat and functionalize by chemical processing to produce a pigment that is readily dispersible in an aqueous vehicle.
A need exists for an effective, easy-to-operate, and lower cost process for making stable pigment dispersions containing organic color pigments as colorant. The present disclosure satisfies this need by providing a process for making a pigment dispersion by using a coating process to introduce a coating agent with functionality to the pigment surface and thus impart the functionality of the coating agent to the pigment surface. Proper selection of coating agents having dispersing moieties results in the coated pigment being dispersible in an aqueous vehicle without the aid of a dispersant.